Two basic approaches to computer-based handwriting recognition are known as on-line handwriting recognition (HWR) and off-line HWR. The off-line HWR approach is sometimes also referred to as optical character recognition (OCR). In general, the on-line HWR technique employs dynamic handwriting information, while the off-line HWR technique employs static handwriting information. These two approaches have historically been separately developed for use in different handwriting recognition environments.
For the case where a handwriting input device provides dynamic information, such as stroke direction and stroke order, the use of on-line HWR technique has traditionally been preferred because, for a given set of input characters, on-line HWR generally provides better recognition accuracy than the off-line HWR technique. A stylus-based (or pen-based) computer that captures strokes from an integrated digitizer tablet is one exemplary application that has benefitted from the use of on-line HWR.
The off-line HWR technique, on the other hand, has traditionally been employed for applications where dynamic information from a digitizer was not available. By example, when documents are scanned from a scanner the input data stream does not contain dynamic handwriting information. As such, the on-line HWR approach is unsuitable for use, and the off-line HWR approach provides a viable alternative.
While the on-line HWR technique provides superior results when dynamic stroke information is available, it has drawbacks as well. For example, the on-line HWR technique can exhibit a sensitivity to user-dependent letter formation. That is, the on-line HWR technique operates best with a well-formed presentation of the constituent strokes of a letter or numeral. The on-line HWR technique may thus exhibit a sensitivity to the order of formation and the number of constituent strokes. For example, the on-line HWR technique typically expects a user to write the letter "C" with a single touch of the pen and with a counter clock-wise stroke. This is, however, but one possible way for a user to form the letter "C". For example, a multi-stroke, clockwise pen motion can also be used.
Often complicating the problem presented by user-dependent character formation is the unnaturalness that many users experience when writing with a conventional stylus/digitizer tablet. Because the feel of the stylus on the surface of the digitizer tablet may not replicate the feel of a pen or pencil on paper, and in that the visual feedback is different than what the user is accustomed to, the resultant input characters may be ill-formed, or formed with an unnatural stroke sequence and/or direction. The retracing of strokes by the user is particularly troublesome for the dynamic HWR technique to properly handle. The end result can be the occurrence of recognition errors when using the on-line HWR technique.
The off-line HWR technique has been shown to be more robust when presented with such irregular handwritings, in that dynamic stroke information is not considered. However, and as was noted above, the overall recognition accuracy of the off-line HWR technique is generally less than that obtained when using the on-line HWR technique.